1. Field of the Invention
This invention relates to power supplies and more particularly relates to preventing smoke and smell in a power supply powered by a current limited source.
2. Description of the Related Art
A power supply, sometimes known as a power supply unit or PSU, is a device or system that supplies electrical or other types of energy to a load or group of loads. A power supply, in some embodiments may be configured to convert power in one form to another form, such as converting alternating current (“AC”) power to direct current (“DC”) power. The regulation of power supplies is typically done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is typically closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input.
For example, in an electronic device such as a computer, the power supply is typically designed to convert an AC voltage input such as is traditionally provided by a conventional wall socket, into several low-voltage DC power outputs for transmission to the internal components of the electronic device. Conversion is typically performed in stages that may include various stages such as a rectification stage, a pre-regulation stage such as an active harmonic filter, a regulator/chopper stage, etc. The stages may be configured in various topologies such as a boost stage, a buck stage, or other derivative topology.
A switching power supply incorporates a switching regulator within one or more stages of the power supply to regulate the output voltage of the power supply. A switching regulator typically includes a switch such as a metal-oxide-semiconductor field-effect transistor (“MOSFET”) that is turned on and off to regulate the output voltage. One common type of fault in switching power supplies is that a short circuit occurs across a switching MOSFET causing the power supply to draw too much current which can result in smoke or even fire.
Power supplies are typically fitted with a current protection device, such as a fuse, so that in the event of a short circuit, the current protection device will trigger thereby preventing smoke or fire from occurring. However, in some cases, such as when a current limited source is used to provide input power to the power supply, it is possible for a short circuit to occur that fails to properly trigger the current protection device. For example, suppose the current limited source is limited to provide less current than is necessary to trip a fuse. In such a case, even though not enough current is being drawn to trip the current protection device, the short circuit may still cause too much current to flow through vulnerable components in the power supply which may result in smoke and smell.
For example, conventional power supplies may utilize a negative temperature coefficient (“NTC”) device in parallel with a relay switch at an input to a switching regulation stage of a power supply. An NTC device is a device that decreases in electrical resistivity as the temperature of the device increases (i.e. higher current results in lower resistance). At startup, the relay switch is in an open position and all of the current passes through the NTC device until the output voltage reaches some threshold. The NTC device acts to slow the input current to the power supply at startup to prevent damage to the power supply components.
Once the output voltage reaches an acceptable threshold, the relay is closed so that substantially all of the current passes through the relay, the relay switch having less resistance than the NTC device. In the event, of a MOSFET short circuit, the relay switch will re-open and substantially all of the current will again pass through the NTC device. However, because of the MOSFET short-circuit too much current will pass through the NTC device as it decreases in resistance causing the device to emit smoke and smell. The present invention resolves the problems associated with the prior art.